In the known ignition system utilizing a magneto, an ignition starting transistor, which is connected in series with the primary winding, is switched to the conductive state at the beginning of the positive voltage half wave in the primary winding of the ignition coil. Primary current therefore flows, which causes a strong magnetic field to be generated in the armature of the magneto generator. When the voltage reaches the maximum value, a threshold element breaks down. This activates a control switch which in turn blocks the ignition starting transistor, thereby interrupting the primary current. A high voltage is therefore induced in the secondary winding of the ignition coil causing a spark to be formed at the spark plug connected thereto.
To give the system the capability of changing the ignition timing, a first and second voltage half wave are generated in the primary circuit. The amplitude of the second half wave is larger than that of the first. At lower engine speeds, the threshold element which causes the initiation of the ignition process responds only to the half wave having a large amplitude. Since, for increasing engine speeds, the induced voltages in the primary circuit also increase, the threshold element will respond to the smaller half wave in the higher speed ranges. The ignition timing is therefore changed by a predetermined amount, that is advanced by a predetermined amount, when the engine speed reaches a predetermined speed.
Ignition systems, and, particularly, ignition systems used in smaller combustion engines, should be kept at as low in price and require as little space as possible. Therefore, for generating the ignition energy, a permanent magnet is utilized which is rotated by means of a fly wheel or a fan wheel which, in turn, is driven by the engine. The pole shoes of the magnet interact with an armature fastened to the motor housing and having a U shaped or E shaped iron core. In order to generate the above mentioned two sequential positive half waves, the primary voltage must be rectified by power diodes. Further, since the first half wave must be smaller than the second, damping circuits are required, which, in some instances, serve to determined the predetermined speed at which the ignition time is advanced.
It is a further disadvantage of the known system, that the ignition angle, that is the angle by which the ignition timing is advanced when the predetermined engine speed is reached, is, to a great extent, determined by the dimensions of the iron core and of the pole shoes of the permanent magnet and therefore cannot be readily adapted to the requirements of the individual engine.